Our previous studies showed that repeated 20R head and neck x-ray exposures significantly enhanced the carcinogenic action of 7,12-dimethylbenz(a)anthracene (DMBA) in Syrian hamster cheek pouch epithelium (HCPE). We proposed that radiation-induced alterations in DMBA and DMBA metabolite binding to target cell DNA is a likely and significant mechanism in this enhancement. The objectives of the proposed studies are to measure the effects of single and split, low to moderate, x-ray doses on the binding of [3H]DMBA and its metabolites to HCPE-DNA in vivo and in organ culture. Initial studies of this possible mechanism during the past two years have established methodology for isolating and purifying HCPE-DNA, measuring total 3H-DMBA binding to DNA, growing HCP in organ culture, and digesting [3H]DMBA-labeled DNA for detailed chromatographic analysis. Preliminary baseline studies have established dose-response and binding vs. time relationships for binding of [3H-DMBA] to HCPE-DNA in vivo. We are also in the midst of similar studies on HCP in organ culture. Proposed studies will examine the effects of varying the x-ray dose, splitting the x-ray dose, and varying the temporal relationship between x-ray and DMBA treatments on chemical and functional parameters of [3H]-DMBA-DNA adduct formation. Paired studies will be done on HCP, both in vivo and in organ culture, to test the applicability of in vitro results to the in vivo situation. We will study the chemistry of the DMBA metabolite-DNA interactions by high-performance liquid chromatography (HPLC) analyses of digested DNA fragments from pouches treated with [3H]DMBA with and without radiation exposures, and we will characterize the functional nature of the DNA in which the adducts form by renaturation kinetic analyses of heat denatured DNA fragments from HCPE treated in vivo or in vitro with [3H]DMBA with and without radiation exposures. The results of these studies may explain a mechanism by which diagnostic levels of x-radiation could enhance carcinogenesis by a variety of chemical agents and thus could provide useful information in the improving qualitative and quantitative accuracy of human radiation risk determination.